Certain optical fiber waveguides exhibit the property of photosensitivity that provides a practical means for photoinducing permanent refractive index changes in the core of those fibers. Photosensitivity is not restricted to fiber structures; it has been detected in several types of planar glass structures, including, for example silca-on-silicon and ion-implanted silca waveguide devices.
The fabrication of optical waveguide devices such as intra-mode retro-reflecting Bragg gratings, mode converter gratings, and rocking rotators have been achieved. The general approach for making these devices is to photoinduce a refractive index grating in the photosensitive core of the optical waveguide. The grating consists of a periodic modulation of the core refractive index along the length of the waveguide. This and other relevant background description relating to methods of photoinducing gratings in photosensitive optical fiber waveguides is well described in U.S. Pat. No. 5,367,588 in the name of Hill et at. issued Nov. 22, 1994. A general overview of photosensitive optical fibers is found in a paper by Raman Kashyap entitled "Photosensitive Optical Fibers: Devices and Applications" published in Optical Fiber Technology 1,17-34 (1994).
Bandpass filters have been fabricated in optical fibers in the form of stepped chirped Bragg gratings. Although these gratings produce bandpass filtering, they are limited in their response and in some instances not suitable.
However, it is believed that as optical fiber grating technology improves, for example to a state where greater refractive index differences are achievable between high low pairs, that more practicable results will be attainable utilizing the basic teachings of this invention. It is further believed, that less ripple and steeper slopes will be attainable utilizing the teachings of this invention.
In view of the limitations of the prior art, it is an object of this invention to provide a bandpass filter that overcomes many of these limitations.
Furthermore, it is an object of this invention to provide a bandpass filter in the form of a Bragg grating multi-Fabry Perot structure that reduces transmission ripple in the passband normally associated with other bandpass filters.
In accordance with the invention, a bandpass filter is provided that comprises an optical waveguide having at least a Bragg grating therein, the at least Bragg grating defining a plurality of Fabry Perot cavities disposed along the waveguide, a first cavity including two reflectors, each reflector comprising "v" alternating high-low (or low-high) index regions within the waveguide, each high or low index region having a thickness of one quarter-wave at a bandpass wavelength, each alternate region being of a different index of refraction than an adjacent region, and a multiple quarter-wave high or low index region sandwiched between the two reflectors, and, a second Fabry Perot cavity including two reflectors, each reflector comprising "u" alternating high-low (or low-high) index regions within the waveguide, each region having a thickness of one quarter-wave at a bandpass wavelength, each alternate region being of a different index of refraction than an adjacent region, and a multiple quarter-wave high or low index region sandwiched between the two reflectors, the values of "u" and "v", may be different and selected in accordance with the refractive index difference between two alternate adjacent regions.
In accordance with the invention, there is provided, a bandpass filter comprising: an optical fiber having Bragg grating, the Bragg grating defining multiple Fabry Perot cavities disposed along the fiber, each cavity comprising a quarter-wave or multiple odd-quarter-wave, at a bandpass wavelength, high or low refractive index a spacer region sandwiched between two reflectors, each reflector comprising a plurality of quarter-wave high/low (or low/high) index periodically alternating regions.